Natural gas coming from a producing well contains many natural gas liquids (NGLs) that are commonly removed. The removal of natural NGLs usually takes place in a relatively centralized processing plant. The objective is to reduce the hydrocarbon dew point to prevent problems in the pipelines from liquid fallout. To remove NGLs, there are three common processes; Refrigeration, Lean Oil Absorption and Cryogenic.
With Refrigeration, a refrigeration plant is employed to provide cold to lower the temperature of the natural gas. Refrigeration is able to extract a large percentage of propane and most of the butane and heavier components.
With Lean Oil Absorption, an absorbing oil with an affinity for NGLs is brought into contact with natural gas in a contact tower where it soaks up a high proportion of NGLs. The “rich” absorption oil, now containing NGLs exits the absorption tower. This “rich” mixture of absorbing oil and NGLs is chilled to −30 F to separate the NGLs and absorbing oil. This process can extract 90% of the propane and heavier hydrocarbons and about 30% of the ethane.
The cryogenic process enables higher recoveries of ethane. The first generation cryogenic plants were able to extract up to 70% of the ethane from the gas, since the early 1990s, modifications to the cryogenic process have allowed ethane recoveries up to 99% extraction level. This increase in recovery comes with higher operating costs. There are a number of different ways to chill the gas the one most commonly used is the turbo expander process. In this process external refrigerants are used to cool the natural gas stream, then an expansion turbine is used to rapidly expand the chilled gases, which causes the temperature to drop significantly. This rapid temperature drop condenses ethane and other hydrocarbons in the gas stream while maintaining methane in a gaseous form. Operations of gas processing plants in reduced recovery modes is difficult, the plants are typically designed to achieve high recoveries of all the NGLs and are not designed to recover only pentanes and heavier or only butanes.